Previous strep B exposure appeared to boost antibody response

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An investigational vaccine for preventing transmission of group B streptococci from mother to newborn was immunogenic for all serotypes, and antibody transfer to neonates was at similar levels to other maternally administered polysaccharide vaccines.

Note that while the results showed both antibody induction in the mother and transfer to the neonate, effects on clinical outcomes were beyond the study's scope.

A vaccine for preventing transmission of group B streptococci from mother to newborn was generally safe and at least somewhat effective among certain groups compared to placebo, a small phase II clinical trial showed.

The vaccine appeared to work better when women already had detectable levels of strep B antibodies, whereas responses were less pronounced among women with no detectable levels of antibodies prior to immunization, Donders and colleagues wrote in Obstetrics & Gynecology.

GBS in neonates is generally treated with intrapartum antibiotics when the mother tests positive for GBS during pregnancy. But as the authors note, because 25%-30% of women are carriers of GBS, this means a portion of healthy women also receive prophylactic antibiotics. These antibiotics are also not nearly as effective in preventing late-onset neonate group B streptococcal disease (when an infant is 7-89 days old) as they are when the disease presents immediately after birth.

The authors said a vaccine has the most potential for preventing GBS disease when antibiotics have no or limited effect, or in settings where antibiotics are impractical or cannot be administered.

The observer-blind, randomized trial enrolled 86 pregnant women, 51 of which were given one 0.5 mL, 5 microgram dose of group B streptococcal vaccine containing capsular polysaccharides of serotypes Ia, Ib and III, while 35 were given 0.5 mL of a 0.9% saline solution. Samples were drawn at baseline, as well as 30 and 90 days after initial immunization.

Donders and colleagues said future research should investigate the poor antibody response among women without detectable levels of antibodies at baseline.

Diane Harper, MD, of the University of Louisville School of Medicine, who was not involved with the study, told MedPage Today that while the results showed antibody induction both in the mother and transfer to the neonate, effects on clinical outcomes were beyond the study's scope.

"The results do not prove that mass vaccination of pregnant women will decrease the incidence of streptococcal infant diseases, nor incur any fewer risks than giving antibiotics intrapartum to those women who are culture positive at term," said Harper. "Further testing with clinical infant outcomes will be needed to show clinical efficacy and modeling to determine cost effectiveness of vaccination [versus] antibiotics will be meaningful research."

When examining the effect of the vaccine on infants 3 months after birth, antibody levels were 22%-25% of those measured at birth, though they were still five-fold to eight-fold higher than in the placebo group. Donders and colleagues also noted that this vaccine did not appear to interfere with the infant's diphtheria immunization.

The vaccine was generally well-tolerated: mild to moderate adverse effects were reported in 63% (95% CI 48.1-75.9%) of the vaccine group versus 74% (95% CI 56.7-87.5%) of the placebo group. Maternal adverse events included headache, hemorrhoids, nasopharyngitis, bronchitis, and urinary tract infection. No more than four participants reported the same adverse event. There were no maternal or infant deaths, and all women gave birth to singleton infants.

Among infants, 24% of the vaccine and 31% of the placebo group reported serious adverse events, with one event (neonatal asphyxia occurring 28 days after maternal vaccination) potentially related to the group B streptococcal vaccination.

Stephanie Schrag, DPhil, of the CDC wrote in an accompanying editorial that the results "signaled new progress" in maternal group B streptococcal evaluation and saw the potential for a vaccine like this in "resource-poor" areas such as sub-Saharan Africa and South Asia.

"Intravenous antibiotic prophylaxis is not feasible in many delivery settings; even where feasible, it can be challenging to screen for indications and implement optimally," she noted.

The idea of a group B streptococcal vaccine has support from other agencies, as well. Schrag cited a recent survey of members of the American College of Obstetricians and Gynecologists (ACOG), which suggested most OB/GYNs would recommend such a vaccine to their patients if it were "proven safe and effective" and endorsed by the CDC.

This study was supported by Novartis Vaccines and Diagnostics, whose non-influenza human vaccines has now transferred to the GlaxoSmithKline (GSK) group of companies.

Donders received research fees for undertaking this trial. Other investigators reported relationships with Novartis and other commercial entities.

Schrag has served as an unpaid member of a data safety monitoring board convened by Novartis for other group B streptococcal vaccine trials.

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